Balanced frequency discriminator



Aug. 19, 1947.

E. BARD ET AL BALANCED FREQUENCY DISCRIMINATOR Filed Oct. 27, 1943jfzql 1. 0

OSCILLATOR MIXER MIXER OSCILLATOR FREQUENCY DISCRIMINATOR D" FREQUENCYLl :fimscmmmon 16 2 Sheets-Sheet l l l l l 1 l I l I x I I l f L f AfDETECTOR BALANCED FREQUENCY CONTROL CHARACTER\STIC CHARACTERISTlCELLIOTTBARD JIIAMZJUIJUKOBZLJKY INVENTORS Aug. 19, 1947. BARD ET'AL2,425,981

BALANCED FREQUENCY DI5CRIMINATOR Filed on. 27,1943 2 Sheets-Sheet 2REACTA NCE OSCILLATOR TU B E LOWER REFERENCE FREQUENCY OSCILLATOR I i i'l E I 20 I/ i 64 g Y 12 '1 M I i 22 HIGHER REFERENCE i I \DII" BALANCEDFREQUEN I CY FREQUENCY OSCILLATOR I f f 6'6 DETECTOR 72 v [74 /76' /7g/80 f9! PbwER DRIVER F.M. REACTANCE OUTPUT DOUBLER '5EPARATR OSQLLATORTUBE 7 J61 LOWER REFERENCE Ev FREQUENCY OSCILLATOR f i i i 20 MODULATORI 10 l 56 i j r 121 i E 1 l "I HIGHER REFERENCE I 1! }-\BALANCEDFREQUENCY FREQUENCY osc|LLATBR f I 55 DETECTOR EZLIQTBARDl/IANUELJULIOFOBIZSKY INVENTORS ATTORNEY Fatented Aug. 19, 1947 BALANCEDFREQUENCY DISCRIMINATOR Elliott Bard and Manuel Julio Kobilsky, BuenosAires, Argentina, assignors to Hartford National Bank and Trust Company,Hartford, Conn,

trustee Application October 27, 1943, Serial No. 507,844

6 Claims.

This invention relates to improvements in frequency detecting systemsand more particularly to balanced frequency detectors especially usefulin frequency stabilizing circuits.

As known in the art, the main disadvantage of the common frequencydiscriminator resides in the critical adjustment of the tuned circuitsthereof and in order to avoid this drawback, discriminators of theso-called frequency counter type have been developed which are fullyexplained in the prior U. S. patent applications of M. Zeigler and M. J.Kobilsky, Serial Number 464,380, filed November 3, 1942, Patent Number2,406,309, dated August 20, 1946, and M. Zei1er,- Serial Number 477,990,filed March 4, 1943. The combination of a frequency counter with a mixertube and a fixed reference oscillator particularly useful in frequencystabilizing systems, constitutes an unbalanced frequency detector whichis easily adjusted and which by utilizing compensating voltage, willprovide a zero output for a predetermined frequency independently of thevariations in the supply voltages.

However, due to the particular frequency transmission characteristic ofthe mixer device, the unbalanced frequency detector comprising afrequency counter provides a zero output voltage for a plurality offrequencies, so that in a frequency stabilizing system the frequencydiscriminator curve thereof crosses the frequency control curve of thesystem to be stabilized at several points which constitute workingpoints of stable equilibrium, and it has been found in practice thatsudden changes in the operating conditions may shift the frequencydetector from one working point to another, thus upsetting the normaloperation of the whole frequency stabilizing system.

In the prior U. S. patent application of M. Zeigler Serial No. 488,582filed May 26, 1943, a frequency stabilizing system has been described,which comprises an unbalanced frequency detector system but in which therange of the frequency control has been effectively restricted, by alimiter stage inserted between the frequency discriminator or counterand the reactance tube thereby limiting the operation of the frequencystabilizing system to one stable working point only.

We have found however, that by using two unbalanced frequency detectorsin a balanced arrangement and by connecting their out-puts in push-pullor opposition, a frequency detector is obtained which will provide azero output voltage for solely one predetermined frequency and thefrequency detector curve thereof, when used in a frequency stabilizingsystem, will cross the frequency control curve at one point only. In thebalanced frequency detector according to the present invention thevariable frequency is simultaneously balanced with respect to a lowerand a higher fixed reference frequency, respectively, by mixing it withsaid reference frequencies so as to obtain a first and a seconddifference frequency respectively. These difference frequencies are thenseparately applied to frequency discriminators or counters which developin their output circuits D. C. voltages proportional to the frequency ofsaid difference frequencies. By connecting the output circuits of bothfrequency discriminators or counters in opposition or pushpull, aresulting balanced detector output voltage is obtained which, beingequal to the difference of the two D. C, voltages, is zero when thevariable frequency corresponds to the mean value of the referencefrequencies, and which is proportional in magnitude and sign to thedeviation of the variable frequency from said mean value.

Therefore the main object of the present invention is to provide abalanced frequency detector which will have a zero output voltage forsolely one predetermined frequency value.

A further object of the present invention is to obtain a frequencydetector, the detector characteristic of which will be independent ofthe variations in the supply voltages and/or circuit parameters, due tothe balanced arrangement of the components thereof and also to itspush-pull output circuit.

A still further object of the present invention is to provide a balancedfrequency detector which will have twice the sensitivity of theunbalanced detector of the frequency counter type.

Another object, of the present invention is to provide a balancedfrequency detector which, when used for frequency stabilizationpurposes, will furnish a stabilizing system having one working pointonly.

These and other objects and advantages of the present invention willbecome apparent from the course of the following description, when readin conjunction with the accompanying drawings, which illustrate onepreferred embodimentof the invention, by way of example.

In the drawings:

Fig. 1 is a connection diagram of the basic layout of a balancedfrequency detector according to the present invention. a

Fig. 1a is a graph illustrating the relative position of the detectorcharacteristic of'the balanced frequency detector with respect to thefixed reference frequencies and also with respect to the frequencycontrol characteristic curve of the stabilizing system shown in Fig. 3.

Fig. 2 is a circuit diagram of the balanced frequency detector accordingto the present invention.

Fig. 3 is a connection diagram illustrating a frequency stabilizingsystem using a balanced frequency detector according to the presentinvention.

Fig. 4 is a connection diagram illustrating a frequency modulationtransmitter stabilized by means of a balanced frequency detectoraccording to the present invention.

The same reference characters are used to indicate like or correspondingparts or elements throughout the drawings.

Referring now to Fig. 1, it may be seen that the balanced frequencydetector according to the present invention comprises a crystalcontrolled oscillator O generating a lower fixed reference frequency fand an oscillator O which is also controlled by a crystal and whichgenerates a higher reference frequency f, oscillators O and and O" arecoupled to one of the inputs of the mixers M and M respectively, theother inputs of which are connected in parallel and comprise the inputterminals l9 and i2 of the balanced frequency detector, to which thevariable frequency .f is applied.

Due to the abovementioned arrangement, mixer M develops in its outputcircuit a first difference frequency Fa while mixer M" generates asecond difference frequency f"u' the output circuits of said mixers Mand M being coupled to the frequency discriminators or counters D and Drespectively, which transform said difference frequencies fd and 1"dinto D. C. voltages v' and 'v proportional to the frequency fd and ,fd'respectively.

In view of the fact that the output terminals I4, l6 and l6,l8 of thediscriminators D and D" respectively are connected in opposition orpushpull, the detector output voltage V developed between outputterminals 20 and 22 of the balanced frequency detector will be equal tothe diner- .ence of said voltages u and v" and will also be proportionalin magnitude and sign to the deviation of the variable frequency J fromthe mean value'of both reference frequencies f and f, as will beexplained hereinafter.

' Considering the individual response characteristics of the frequencydiscriminators or counters D and D, we can write for the voltages vandv" ,where I'd and fd represent the difference frequencies generatedby the mixers M and M and ki, k'z, 76's and 10"1, 70"2 and. it"srepresent the coefficients of the transfer characteristics of thediscriminators D and D" respectively.

As both voltages v and o are connected in opposition, detector voltage Vwill be equal to the difference of v and c", or

' +7c3.[(fd) (f"d) W e k1=7c'1=k"1; k2=k'2=k2; k3=k3:k"3 since bothdiscriminators or frequency counters D and D" are arranged to have liketransfer characteristics.

Considering Equation 3 it will be evident to those skilled in the artthat the detector voltage V, developed between the output terminals 20and 22 of the balanced frequency detector will be zero only when thedifference of the frequencies f'd and fe is zero, viz. when the variablefrequency f corresponds to the mean frequency of the fixed referencefrequencies f and f. Consequently. the balanced frequency detectoraccording to the invention will have a Zero detector voltage only whenthe variable frequency f passes through a point equidistantly locatedbetween the reference frequencies f and 1", as may be seen in the graphshown in Fig. 1a.

Since in Equation 3 the higher order terms are of much smaller magnitudethan the linear term of the difference frequencies fa and fd, adeviation of the variable frequency 1 from its central or mean value fmwill produce between the output terminals 28 and 22 of the balanceddetector a voltage V=2K(j), where K represents the transfer constant ofthe balanced frequency detector and f=(fafa) represents the frequencydeviation. Thus the D. C. detector voltage V of the balanced frequencydetector will not only be proportional in magnitude to the frequency 1but will be either positive or negative depending on whether thevariable frequency 1' is above or below the mean frequency fm.

It may be seen in Fig. 1 that oscillator O, mixer M, discriminator D andoscillator O, mixer M, discriminator D" constitute in fact twounbalanced frequency detectors which, due to the location of thereference frequencies f and f with respect to the variable frequency 1,and also due to the push-pull connection of the discriminator outputcircuits, form together a balanced frequency detector which has a zerooutput voltage for the predetermined mean value of the variablefrequency 1.

Fig. 2 represents the circuit of the balanced frequency detector basedon the diagram of Fig. 1, in which input terminal 12 is connected toground potential, while input terminal i0 is connected to the signalgrids 9'1 and g"1 of two thermionic mixer tubes 1 and M respectively,the load resistances 2G and 26 of which are connected to the positivepole of a direct current source ill from which the other supply voltagesfor the other electrodes of the tubes are derived as is known in theart.

Injector grids 9'3 and 41"3 of said mixer tubes M and M are coupled tooscillators O and 0" respectively, constituted by triode tubescontrolled by the corresponding quartz crystals 28 and 30. Oscillator Ogenerates the lower reference frequency 1" whereas oscillator O"generates the higher reference frequency 1, so that in the platecircuits of the mixers M and M the corresponding difference frequencies,fd and f"d are developed. The plate circuits of said mixer tubes M andM are provided with filte condensers 29 and, SI respectively, whichconstitute shortcircuits for the reference frequencies f and i so thatonly the difference frequencies fd and ,f"a are applied to the inputs ofthe discriminators D and D" which, in the present embodiment of theinvention, are constituted by frequency counters of the type describedin the prior U. S. patent application Serial No. 477,990.

Said frequency counters D and D comprise a thermionic counter valve V1and V2 and an integrating circuit constituted by a half-wave dioderectifier V's, V"3 respectively, the frequency .counters being coupledto the corresponding mixers M and M" through coupling condensers 32 and34 and potentiometers P and P formed by the resistances 3E, 38 and 40,42 respectively. The screen grids of the tubes V1 and V2 are connectedthrough a common voltage-dropping resistance 44 to the positive pole ofdirect current supply source 21, to which the plate circuits of bothtubes are also connected. These plate circuits are constituted byresistance 55, self-inductance 48, and resistance 56, a self-inductance52 respectively, inductance 48 constituting the primary winding of atransformer T1, the secondary winding 54 of which, together with a loadresistance 56 and the half-Wave diode rectifier V's formed by anode 58and cathode 68, constitutes the integrating circuit of the frequencycounter D.

Inductance 52, inserted in the plate circuit of the counter tube V2constitutes the primar winding of a coupling transformer T2, thesecondary winding 62 of which is connected in series with loadresistance fi l and the half-wave diode rectifier V"3 comp-rising anode56 and the cathode 69 thus forming the integrating circuit of thefrequency counter D".

The operation of the frequency counters has been fully described in theprior U. S. patent application Serial No. 477,990 Where it has beenexplained that when a sinusoidal voltage is applied to the control gridsG1 or G2 of the counter tubes V1 or V2, the plate current of said tubesflowing through the plate circuits constituted by 48, 46 and 52, 50respectively, will vary for each half period of like sign of the applieddifference frequency so as to generate across the inductances l8 and 52voltage impulses which, when rectified by the half-wave diode rectifiervalves V's and Vs and integrated over a time which is sufficiently largecompared with the period Of the difference frequencies 'd and f"ddevelop across the load resistances 56 and 64 the voltages o and v"proportional to the frequency of said difference frequencies ,f'd and"d.

As may be seen in Fig. 2, the output terminals 20 and 22 of the balancedfrequency detector are connected to the ends of said load-resistances 56and 64, the junction it of which is connected to the common cathode Bil,so that both load resistances form a push-pull output circuit wherein adetector output voltage V is developed which, being equal to thedifference of said voltages v and '0", is proportional to the deviationof the variable frequency 3 from the central or mean frequency in, asexplained hereinbefore.

In short, the balanced frequency detector according to the presentinvention is formed by two separate unbalanced frequency detectors ofthe frequency counter type, each constituted by the reference oscillatorO mixer M (l and counter tube V1 (V2) each unbalanced frequency detectorbeing provided with an integrating circuit the load resistances E and 6of which are connected in push-pull or opposition. In view of thebalanced arrangement of said frequency detectors and due to thepush-pull connection of the frequency counter outputs, a balancedfrequency detector is obtained which provides a zero output voltagewithout the use of a compensating voltage and in which slight variationsof the circuit parameters will be automatically compensated. Moreover,the balanced arrangement will compensate for slight and oppositevariations of the reference frequencies, thus greatly improving theperformance of the novel frequency detector.

The balanced'frequency detector according to the present invention isparticularly adapted for frequency stabilization purposes by applying tothe input thereof the frequency of an oscillator to be stabilized andfeeding back the detector output voltage through a reactance tube to theoscillator, the frequency of which should of course be approximately thearithmetic mean of the two fixed reference frequencies 1" and f.

The circuit arrangement for this purpose is shown in block diagram inFig. 3, in which the balanced frequency detector is represented by therectangle 55, and in which the output voltage V of the frequencydetector is applied to a, reactance tube or equivalent 'device 68 whichcontrols the frequency of an oscillator 10 which in turn is coupled tothe input terminals ill, l2 of the balanced frequency detector.

Those skilled in the art will readily understand that in the frequencystabilizing system shown in Fig. 3, the frequency of the oscillator '10will tend automatically to adjust itself very near to the central ormean frequency fm of the reference frequencies J" and 1, since everydeviation of the oscillator frequency from fm will produce a corresponding voltage V in the output circuit of the balanced frequencydetector 66, and said voltage, acting through reactance tube 68, willtend to readjust the oscillator 16 to its original frequency.

A low pass filter may be included in the output circuit of the balancedfrequency detector, to exclude from the output voltage the components ofthe frequency of the voltage impulses and bar monies thereof, so as toobtain solel a voltage which will vary proportionally to the deviationof the frequency ,f to be controlled.

In view of the fact that the detector output volta e V has a zero valuefor only one predetermined frequency, the above described frequencystabilization system has only one Working point, viz. the detectorcharacteristic and the frequency control curves cross at one point only,as may be observed in Fig. 1a.

As illustrated in Fig. 4, the novel balanced frequency detector may bealso applied to frequency modulated transmission. The transmitterantenna I2 is connected through a power output section it, a driver ordoubler l8 and a separator it to an oscillator 89 which is alsoconnected to the input of the balanced frequency detector represented byrectangle 68. The output of the balanced detector is applied through anoutput lowpass filter 82 to the reactance tube or similar device 8%which controls the frequency of oscillator to.

The frequency stabilizing circuit of Fig. 4 is substantially that shownin Fig. 3, except for output low-pass filter S2 and modulator 8%connected to the same, so that the balanced frequency detector output,when passin through said lowpass filter 82 is modulated by theintelligence amplified by the modulator 88. The reactance tube 85therefore acts both as stabilize and as modulator, and while thebalanced frequency detector 8E maintains the oscillator near the centralfrequency fm, th output of the oscillator is also frequency modulated bysuperposition of the intelligence derived from modulator 86.

In the described embodiment of the novel balanced frequency detector,frequency discriminators of the frequency counter type have beenutilized which are described in the prior U. S. patent application No.477,990. However, it is to be understood that this invention ,is notlimited t the type of the discriminator ,her nbefore specifically dscribed'for the purposeof illustration and that variations and modifcations may be made without departing from the spirit of this invention,and such variations and modifications, or the use of such individualfeatures or subcombination of featuresas do not depart from the 'erencefrequencies, a second crystal controlled oscillator for generating a'wavehaving the higher of said reference frequencies, first and secondthermionic mixer tubes each having an injector grid, a signal grid andan output'electrode, means to connect said signal grids in parallel andto apply thereto said variabl frequency wave, means to couple said firstand said second oscillators to theinjec'tor grids of said first andsecond mixer tubes respectively, frequency counters coupled to theoutput electrodes of said mixer tubes for deriving a' first and a secondvoltage proportional to the frequency of the difference frequency wavesgenerated in the output circuits of said first and said second mixertubes respectively, each of said frequency counters comprising a directcurrent source, an inductance connected thereto, means to vary thecurrent a like value for each half period of like sign of the respectivedifference frequency wave and thereby generate voltage impulses acrosssaid inductances, and means to integrate the voltage impulses to producea voltage, and means to connect the respective frequency counters inpush-pull to produce an output voltage varying as the difference of therespective derived voltages and which is zero for said mean frequencyvalue of the variable frequency wave and proportional to the deviationof said Variable frequency wave from said mean frequency value.

2. A balanced frequency detector having zero output at a frequency Valueof a variable frequency wave corresponding to the mean frequency of twofixed reference frequencies, comprising a first crystal-controlledoscillator for generating a wave having the ,lower of said referencefrequencies, a second crystal-controlied oscillator for generating awavehaving the higher of said reference frequencies, first and secondthermionic mixer tubes each having an injector grid, a signal grid andan britputjeliectrod, means to'connect saidsignal grids in parallel andto apply thereto said variable frequency wave, means to couple saidfirst and second oscillators to the injector grids of said first andsecond mixer tubes respectively, andfrequenc y counters coupled to theoutput electrodes of said mixer'tubes, each of' said frequency counterscomprising a thermionic tube including cathode, a control grid and ananode, means to couple the control grid to the output electrode of thecorresponding mixer tube, a resistance, a self-inductance and a, currentsource connected in series circuit arrangement with said cathode andanode, and voltage integrating means inductively coupled to ai a o e i ca olta einte ratin means cQmprising a half-wave diode rectifier .tubeanda load resistance coupled thereto, the respective load resistances ofthe frequency counters being interconnected toproduce an output voltagevarying as the difference of the voltages developed across each of theload resistances.

3. A balanced frequency detector having zero output at a frequency valueof a variable frequency wave corresponding to the mean frequency of twoconstant reference frequencies, said detector comprising means forgenerating a wave having the lower of said reference frequencies, meansfor mixing said variable frequency wave with said lower referencefrequency wave to produce a first difference frequency wave, means forgenerating a Wave having the higher of said reference frequencies, meansfor mixing said variable frequency wave with said higher referencefrequency Wave to produce a second difference frequency wave, frequencycounters coupled to the respective mixing means, each of said frequencycounter comprising a thermionic counter tube including a cathode, acontrol grid and an anode, means to couple the control grid to thecorresponding mixing means, a resistance, a self-inductance and acurrent source connected in series circuit arrangement with said cathodeand anode, and voltage integrating means inductively coupled to saidanode circuit, each of said voltage integrating means comprising ahalf-wave diode rectifier tube and a load resistance coupled thereto,the respective load resistances of the frequency counters beingconnected to produce an output voltage varying as the difference of thevoltages developed across each of the load resistances.

4. A balanced frequency detector having zero output at a frequency valueof a variable frequency wave corresponding to the mean frequency of twoconstant reference frequencies, said detector comprising means forgenerating a wave having the lower of said reference frequencies, meansfor mixing said variable frequency Wave with said lower referencefrequency wave to produce a first difference fro-quency wave,

r means for generatin a wave having the higher of in series circuitarrangement with said cathode and anode, a winding'inductively coupledto said self-inductance and a diode rectifier and a load resistancecoupled to said winding, the respective load resistances of thefrequency counters being connected to produce an output voltage varyingas the difference of the voltages developed across each of the loadresistances.

5. A frequency stabilizing apparatus comprising in combination a sourceof electrical oscillations the frequency of which is to be stabilizedand including frequency adjusting terminals and an output circuit, and afrequency detector comprising two mixers each including two inputterminals and an output circuit, and a source of direct current, one ofthe input terminals of said mixers being'connected in parallel totheoutput circuit of said source of electric oscillations, oscillators forgenerating a lower fixed reference frequency .wave and a higher fixedreference frequency wave, means to couple said oscillators one to eachof the other of the input terminals of said mixers, two frequencycounters having input terminal coupled to the output circuits of saidmixers and connected to said source of direct current, each frequencycounter comprising means for impulsively varying said direct current inresponse to the variations of the difference frequency waves produced inthe output circuits of said mixers, a circuit connected to said sourceof current and including said current varying means and aself-inductance, voltage integrating means coupled to saidself-inductance, the output circuits of said voltage integrating meansbeing connected in push-pull and constituting a balanced output circuitfor producing a voltage proportional to the frequency variation of saidsource of electrical oscillations, frequency adjusting means havinginput terminals connected to the output of said balanced output circuit,and output terminals connected to the frequency adjusting terminals ofthe source of oscillations, whereby a correcting adjustment is impressedon said source of oscillations is response to variations in the balancedoutput circuit of the frequency detector.

6. A frequency modulated transmitter comprising in combination an outputcircuit terminating in a transmitting antenna, a frequency modulatedoscillator including modulation terminals and output terminals connectedto said output circuit, two mixers each including two input terminals,an output circuit and a source of direct current, one of the inputterminals of each mixer being connected in parallel and. being coupledto said frequency modulated oscillator, oscillators for generating alower reference frequency wave and a higher reference frequency wave,means to couple said oscillators one to each of the other of the inputterminals of said mixers to produce in the output circuits of saidmixers first and second difierence frequency waves, two frequencycounters having input terminals coupled to the output circuits of saidmixer and 10 connected to said source of direct current, each frequencycounter comprising means for impulsively varying said direct current inresponse to variations of the outputs of said mixers, a circuitconnected to said source of current and including said current varyingmeans and a self-inductance, voltage integrating mean coupled to saidself-inductance, the output circuit of said voltage integrating meansbeing connected in pushpull and constituting a balanced output circuitdelivering a. voltage proportional to the frequency deviation of saidsource of oscillations from the mean frequency value of said lower andsaid higher fixed reference frequency waves, a source of voltageproportional to intelligence to be transmitted, a band-pass filterelement connected to said intelligence voltage source and to saidbalanced output circuit, and frequency adjusting means connected betweenthe output terminals of said band-pass filter and the modulationterminals of said frequency modulated oscillator.

ELLIOTT BARD. MANUEL JULIO KOBILSKY,

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